Atomic-level polarization reversal in sliding ferroelectric semiconductors.

Intriguing "slidetronics" has been reported in van der Waals (vdW) layered non-centrosymmetric materials and newly-emerging artificially-tuned twisted moiré superlattices, but correlative experiments that spatially track the interlayer sliding dynamics at atomic-level remain elusive. Here, we address the decisive challenge to in-situ trace the atomic-level interlayer sliding and the induced polarization reversal in vdW-layered yttrium-doped γ-InSe, step by step and atom by atom. We directly observe the real-time interlayer sliding by a 1/3-unit cell along the armchair direction, corresponding to vertical polarization reversal. The sliding driven only by low energetic electron-beam illumination suggests rather low switching barriers. Additionally, we propose a new sliding mechanism that supports the observed reversal pathway, i.e., two bilayer units slide towards each other simultaneously. Our insights into the polarization reversal via the atomic-scale interlayer sliding provide a momentous initial progress for the ongoing and future research on sliding ferroelectrics towards non-volatile storages or ferroelectric field-effect transistors.

Bevel-edge epitaxy of ferroelectric rhombohedral boron nitride single crystal.

Within the family of two-dimensional dielectrics, rhombohedral boron nitride (rBN) is considerably promising owing to having not only the superior properties of hexagonal boron nitride1-4-including low permittivity and dissipation, strong electrical insulation, good chemical stability, high thermal conductivity and atomic flatness without dangling bonds-but also useful optical nonlinearity and interfacial ferroelectricity originating from the broken in-plane and out-of-plane centrosymmetry5-23. However, the preparation of large-sized single-crystal rBN layers remains a challenge24-26, owing to the requisite unprecedented growth controls to coordinate the lattice orientation of each layer and the sliding vector of every interface. Here we report a facile methodology using bevel-edge epitaxy to prepare centimetre-sized single-crystal rBN layers with exact interlayer ABC stacking on a vicinal nickel surface. We realized successful accurate fabrication over a single-crystal nickel substrate with bunched step edges of the terrace facet (100) at the bevel facet (110), which simultaneously guided the consistent boron-nitrogen bond orientation in each BN layer and the rhombohedral stacking of BN layers via nucleation near each bevel facet. The pure rhombohedral phase of the as-grown BN layers was verified, and consequently showed robust, homogeneous and switchable ferroelectricity with a high Curie temperature. Our work provides an effective route for accurate stacking-controlled growth of single-crystal two-dimensional layers and presents a foundation for applicable multifunctional devices based on stacked two-dimensional materials.

An inorganic liquid crystalline dispersion with 2D ferroelectric moieties.

Electro-optical effect-based liquid crystal devices have been extensively used in optical modulation techniques, in which the Kerr coefficient reflects the sensitivity of the liquid crystals and determines the strength of the device's operational electric field. The Peterlin-Stuart theory and the O'Konski model jointly indicate that a giant Kerr coefficient could be obtained in a material with both a large geometrical anisotropy and an intrinsic polarization, but such a material is not yet reported. Here we reveal a ferroelectric effect in a monolayer two-dimensional mineral vermiculite. A large geometrical anisotropy factor and a large inherent electric dipole together raise the record value of Kerr coefficient by an order of magnitude, till 3.0 × 10-4 m V-2. This finding enables an ultra-low operational electric field of 102-104 V m-1 and the fabrication of electro-optical devices with an inch-level electrode separation, which has not previously been practical. Because of its high ultraviolet stability (decay <1% under ultraviolet exposure for 1000 hours), large-scale production, and energy efficiency, prototypical displayable billboards have been fabricated for outdoor interactive scenes. This work provides new insights for both liquid crystal optics and two-dimensional ferroelectrics.

Letter to the editor.

We review the study by Xu et al. (J Clin Monit Comput 37(4):985-992, 2023. https://doi.org/10.1007/s10877-022-00968-1 ) on ultrasound-guided regional blocks in clavicle surgery, assessing the effects on anaesthesia and postoperative outcomes. However, there are concerns. The defined population of the study differs from the registered title (Xu et al. J Clin Monit Comput 37(4):985-992, 2023. https://doi.org/10.1007/s10877-022-00968-1 ). In particular, the authors did not specify fracture types (Xu et al. J Clin Monit Comput 37(4):985-992, 2023. https://doi.org/10.1007/s10877-022-00968-1 ). In addition, the method of measuring the diaphragm is not clear (Xu et al. J Clin Monit Comput 37(4):985-992, 2023. https://doi.org/10.1007/s10877-022-00968-1 ). This affects the accurate interpretation of their results.

Atypical Sliding and Moiré Ferroelectricity in Pure Multilayer Graphene.

Most nonferroelectric two-dimensional materials can be endowed with so-called sliding ferroelectricity via nonequivalent homobilayer stacking, which is not applicable to monoelement systems like pure graphene bilayer with inversion symmetry at any sliding vector. Herein, we show first-principles evidence that multilayer graphene with N>3 can all be ferroelectric, where the polarizations of polar states stem from the symmetry breaking in stacking configurations of across layer instead of adjacent layer, which are electrically switchable via interlayer sliding. The nonpolar states can also be electrically driven to polar states via sliding, and more diverse states with distinct polarizations will emerge in more layers. In contrast to the ferroelectric moiré domains with opposite polarization directions in twisted bilayers reported previously, the moiré pattern in some multilayer graphene systems (e.g., twisted monolayer-trilayer graphene) possess nonzero net polarizations with domains of the same direction separated by nonpolar regions, which can be electrically reversed upon interlayer sliding. The distinct moiré bands of two polar states should facilitate electrical detection of such sliding moiré ferroelectricity during switching.

Exploitation of mixed-valency chemistry for designing a monolayer with double ferroelectricity and triferroic couplings.

Mixed-valence compounds possess both intriguing chemical and physical properties such as the intervalence charge transfer band and thus have been excellent model systems for the investigation of fundamental electron- and charge-transfer phenomena. Herein, we show that valence stratification can be a source of symmetry breaking and generating ferroelectricity in two-dimensional (2D) materials. We present ab initio computation evidence of the monolayer Cu2Cl3 structure with Cu ions being stratified into two separated layers of Cu(I) and Cu(II). Chemically, this unique monolayer not only entails lower formation energy than the bulk CuCl + CuCl2, but also enables the swapping of two valences through vertical ferroelectric switching, leading to a hitherto unreported chemical valencing phenomenon. Notably, the Jahn-Teller distortion of the Cu(II) layer results in another source of symmetry breaking and thus in-plane ferroelectricity. Apart from the valence swapping and self-contained double ferroelectricity, the monolayer's ferroelasticity is also coupled with in-plane ferroelectricity, while the monolayer's ferromagnetism is coupled with vertical polarization owing to the distinct magnetization of each Cu(I) and Cu(II) layer, thereby evoking the long-sought 2D triferroicity as well as triferroic couplings.

Two-Dimensional Ultrahigh Unconventional Piezoelectricity Driven by Charge Screening.

In the past decade, piezoelectricity has been explored in a series of two-dimensional (2D) materials for nanoelectromechanical applications, while their piezoelectric coefficients are mostly much lower than those of prevalent piezoceramics. In this paper, we propose an unconventional approach of inducing 2D ultrahigh piezoelectricity dominated by charge screening instead of lattice distortion and show the first-principles evidence of such piezoelectricity in a series of 2D van der Waals bilayers, where the bandgap can be remarkably tuned via applying a moderate vertical pressure. Their polarizations can switch between the screened and unscreened state by a pressure-driven metal-insulator transition, which can be realized via tuning interlayer hybridization or inhomogeneous electrostatic potential by substrate layer to change the band splitting or tuning the relative energy shift between bands utilizing the vertical polarization of the substrate layer. Such 2D piezoelectric coefficients can be unprecedented and orders of magnitude higher than those of previously studied monolayer piezoelectrics, and their high efficiency of energy harvesting in nanogenerators can be expected.

Covalent-like bondings and abnormal formation of ferroelectric structures in binary ionic salts.

In the Mooser-Pearson diagram, binary ionic compoundss form into nonpolar symmetrical structures with high coordination numbers, while wurtzite structures should appear in the covalent region. Their tetrahedral bonding configurations break the inversion symmetry, with polarizations almost unswitchable due to the high barriers of abrupt breaking and reformation of covalent bonds. Here, through first-principles calculations, we find some exceptional cases of highly ionic ferroelectric binary salts such as lithium halides, which may form into wurtzite structures with covalent-like sp3 bondings, and the origin of these abnormal formations is clarified. Their high polarizations induced by symmetry breaking are switchable, with much smoother switching pathway refrained from abrupt bond breaking due to the long-range feature of Coulomb interactions. These covalent-like ionic bondings do reduce not only their ferroelectric switching barriers but also the phase transition barriers between polar and nonpolar phases, rendering high performance in applications such as nonvolatile memory and energy storage.

Unconventional Ferroelectricity with Quantized Polarizations in Ionic Conductors: High-Throughput Screening.

Ferroelectricity is generally a displacive phenomenon within a unit cell in which ions are placed asymmetrically. In ionic conductors, ions can also be electrically displaced but by much longer distances. They are mostly nonpolar with symmetrical lattices due to the nondirectional character of ionic bondings. Here we propose that the combination of two such displacive modes may give rise to unconventional ferroelectricity with quantized polarizations, where even one local vacancy may induce giant polarization in ubiquitous ionic conductors. Such systems should be insulating with ion vacancies inclined to aggregate at one side. Our high-throughput screening combined with ab initio calculations provided 35 candidates, from which we select KSnS4 and Na4SnS4 to show the existence of such long ion displacement ferroelectricity with a change in integer quantum number in polarizations during switching. The polarizations can be unprecedentedly large with a moderate density of ion vacancies that can be experimentally achieved via ion deintercalation.

Giant ferroelectric polarization in a bilayer graphene heterostructure.

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 1013 cm-2, which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m-1 is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena.

Spatially Resolved Polarization Manipulation of Ferroelectricity in Twisted hBN.

Robust room-temperature interfacial ferroelectricity has been formed in the 2D limit by simply twisting two atomic layers of non-ferroelectric hexagonal boron nitride (hBN). A thorough understanding of this newly discovered ferroelectric system is required. Here, twisted hBN is used as a tunneling junction and it is studied at the nanometer scale using conductive atomic force microscopy. Three properties unique to this system are discovered. First, the polarization dependence of the tunneling resistance contrasts with the conventional theory. Second, the ferroelectric domains can be controlled using mechanical stress, highlighting the original meaning of the emergent "slidetronics". Third, ferroelectric hysteresis is highly spatially dependent. The hysteresis is symmetric at the domain walls. A few nanometers away, the hysteresis shifts completely to the positive or negative side, depending on the original polarization. These findings reveal the unconventional ferroelectricity in this 2D system.

High-TC Two-Dimensional Ferroelectric CuCrS2 Grown via Chemical Vapor Deposition.

Two-dimensional (2D) ferroelectrics have attracted intensive attention. However, the 2D ferroelectrics remain rare, and especially few of them represent high ferroelectric transition temperature (TC), which is important for the usability of ferroelectrics. Herein, CuCrS2 nanoflakes are synthesized by salt-assisted chemical vapor deposition and exhibit switchable ferroelectric polarization even when the thickness is downscaled to 6 nm. On the contrary, a CuCrS2 nanoflake shows a TC as high as ∼700 K, which can be attributed to the robust tetrahedral bonding configurations of Cu cations. Such robustness can be further clarified by a theoretically predicted high order-disorder transition barrier and structure evolution from 600 to 800 K. Additionally, the interlocked out-of-plane (OOP) and in-plane (IP) ferroelectric domains are observed and two kinds of devices based on OOP and IP polarizations are demonstrated.

Optimal prophylactic para-aortic radiotherapy in locally advanced cervical cancer: anatomy-based versus margin-based delineation.

OBJECTIVE: Precise delineation of the para-aortic nodal region is critical for the optimal therapeutic ratio of prophylactic para-aortic radiotherapy. We aimed to evaluate the para-aortic control and patient-reported gastrointestinal toxicity in patients with locally advanced cervical cancer who received anatomy-based or margin-based prophylactic para-aortic radiotherapy. METHODS: We analyzed 160 patients with locally advanced cervical cancer who received prophylactic extended-field radiotherapy between January 2014 and November 2019 at two tertiary centers. Para-aortic nodal regions were delineated based on the anatomic principle-based atlas or marginal expansion from the aorta and inferior vena cava. The Patient-Reported Outcome version of the Common Terminology Criteria for Adverse Events was used to assess acute gastrointestinal toxicity, and a score of ≥3 was defined as severe gastrointestinal toxicity. RESULTS: Seventy-six (47.5%) and 84 (52.5%) patients received anatomy-based and margin-based prophylactic para-aortic radiotherapy, respectively. The median follow-up was 40.1 months (IQR 25.5-58.9). Para-aortic nodal failures occurred in one (1.3%) patient in the anatomy-based para-aortic radiotherapy group and in one (1.2%) patient in the margin-based para-aortic radiotherapy group (p=1.00). There was no in-field or marginal para-aortic nodal failure. The 3-year para-aortic recurrence-free survival for anatomy-based and margin-based para-aortic radiotherapy was 98.6% and 98.8%, respectively (p=0.94). Patients who received anatomy-based para-aortic radiotherapy reported less severe acute gastrointestinal toxicity than those who received margin-based para-aortic radiotherapy (13.2% vs 29.8%, p=0.01). A comparison of gastrointestinal toxicities showed that patients who received anatomy-based para-aortic radiotherapy reported significantly less severe gastrointestinal toxicity than those who received margin-based para-aortic radiotherapy in terms of frequency of diarrhea (7.9% vs 20.2%, p=0.03), severity of abdominal pain (3.9% vs 14.3%, p=0.03), and interference of abdominal pain (2.6% vs 11.9%, p=0.03). CONCLUSION: Anatomy-based prophylactic para-aortic radiotherapy achieved excellent para-aortic control and a lower incidence of severe patient-reported gastrointestinal toxicity. These findings suggest that anatomy-based delineation optimizes clinical outcomes of prophylactic para-aortic radiotherapy in locally advanced cervical cancer.

SbCl4: An Exceptional Superhalogen as the Building Block of a Mixed Valence Supercrystal with Unconventional Ferroelectricity.

Superhalogens are nanoclusters with high electron affinities, exhibiting behavior similar to that of halogens. Their dimerization yields nonpolar symmetrical clusters, akin to diatomic halogen molecules, and they are unstable in the condensed phase in the absence of charge-compensating cations. Herein, we provide ab initio evidence that SbCl4 superhalogen is an exception: its dimerization yields a polar cluster that can be viewed as a quasi-bonded [SbCl5]δ- and [SbCl3]δ+ Lewis acid-base cluster. The symmetry breaking arises from the valence stratification of Sb into Sb5+ and Sb3+ as well as their lone pair electrons. When assembled, SbCl4 clusters form a supercrystal that is thermodynamically stable up to 600 K, with the unique bonding feature of Sb2Cl8 prevailing in the bulk phase. Combination of mixed valence and lone pair electrons leads to electric polarizations along all directions, generating a type of unconventional multimode ferroelectricity in which three different modes of ferroelectricity with distinct magnitudes and Curie temperature are revealed.

A multiferroic vanadium phosphide monolayer with ferromagnetic half-metallicity and topological Dirac states.

Ferroelasticity, ferromagnetism, half-metallicity, and topological Dirac states are properties highly sought in two-dimensional (2D) materials for advanced device applications. Here, we report first-principles prediction of a dynamically and thermally stable tetragonal vanadium phosphide (t-VP) monolayer that hosts all these desirable properties. This monolayer is substantially ferromagnetic with polarized spins aligned in the in-plane direction via a d-p-d super-exchange coupling mechanism; meanwhile, its tetragonal lattice enables an intrinsic in-plane ferroelasticity with a reversible strain of 23.4%. As a result, the ferroelasticity is strongly coupled with ferromagnetism via spin-orbit coupling to enable deterministic control over the magnetocrystalline anisotropy by an applied elastic strain. More interestingly, this multiferroic t-VP monolayer possesses half-metallicity with an anisotropic, topological Dirac cone residing in the majority-spin channel. We also predict a multiferroic t-CrN monolayer, whose ferromagnetism features a high Curie temperature of up to 478 K but is weakly coupled to its in-plane ferroelasticity. These results suggest a tetragonal 2D lattice as a robust atomic-scale scaffold on the basis of which fascinating electronic and magnetic properties can be rationally created by a suitable combination of chemical elements.

Sliding ferroelectricity in 2D van der Waals materials: Related physics and future opportunities.

Near the 100th anniversary of the discovery of ferroelectricity, so-called sliding ferroelectricity has been proposed and confirmed recently in a series of experiments that have stimulated remarkable interest. Such ferroelectricity exists widely and exists only in two-dimensional (2D) van der Waals stacked layers, where the vertical electric polarization is switched by in-plane interlayer sliding. Reciprocally, interlayer sliding and the "ripplocation" domain wall can be driven by an external vertical electric field. The unique combination of intralayer stiffness and interlayer slipperiness of 2D van der Waals layers greatly facilitates such switching while still maintaining environmental and mechanical robustness at ambient conditions. In this perspective, we discuss the progress and future opportunities in this behavior. The origin of such ferroelectricity as well as a general rule for judging its existence are summarized, where the vertical stacking sequence is crucial for its formation. This discovery broadens 2D ferroelectrics from very few material candidates to most of the known 2D materials. Their low switching barriers enable high-speed data writing with low energy cost. Related physics like Moiré ferroelectricity, the ferroelectric nonlinear anomalous Hall effect, and multiferroic coupling are discussed. For 2D valleytronics, nontrivial band topology and superconductivity, their possible couplings with sliding ferroelectricity via certain stacking or Moiré ferroelectricity also deserve interest. We provide critical reviews on the current challenges in this emerging area.

Ultra-high piezoelectric coefficients and strain-sensitive Curie temperature in hydrogen-bonded systems.

We propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically, where ferroelectrics with strain-sensitive Curie temperature are necessary. We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics (e.g. organic PhMDA) can be ideal candidates, which are also flexible and lead-free. Owing to the specific features of hydrogen bonding, their proton hopping barrier will drastically increase with prolonged proton transfer distance, while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds. Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%. Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction, and in another direction, an unprecedented ultra-high piezoelectric coefficient of 2058 pC/N can be obtained. This value is even underestimated and can be greatly enhanced when applying a smaller strain. Aside from sensors, they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients.

Room-Temperature Ferroelectricity in 2D Metal-Tellurium-Oxyhalide Cd7Te7Cl8O17 via Selenium-Induced Selective-Bonding Growth.

Two-dimensional (2D) ferroelectric materials have attracted increasing interest due to meeting the requirements of integration, miniaturization, and multifunction of devices. However, the exploration of intrinsic 2D ferroelectric materials is still in the early stage, for which the related reports are still limited, especially fewer ones prepared by chemical vapor deposition (CVD). Here, the ultrathin metal-tellurium-oxyhalide Cd7Te7Cl8O17 (CTCO) flakes as thin as 3.8 nm are realized via the selenium-induced selective-bonding CVD method. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be ascribed to the induction of selective bonding of a hydrogen atom in H2O molecules by the introduction of selenium, leading to the generation of strong oxidants. Excitingly, switchable out-of-plane ferroelectric polarization was observed in CTCO flakes down to 6 nm at room temperature, which may be caused by mobile Cl vacancies. This work has implications for the synthesis and applications of 2D ferroelectric materials.